Saturday, 29 August 2015

Simple Breadboard Power Supply

It's very satisfying to build something for which you've all the parts to hand, there's no waiting around, just immediate gratification. This little build is such a thing. It's a dual-voltage (5v and 3.3v) power supply which plugs into a breadboard and is handy for projects which aren't powered from a USB tether (e.g., wireless microcontroller prototypes or projects which don't use a microcontroller at all, hard as that might be to believe).

It's loosely based on the circuit below, although I used different values for the capacitors, having first done a bit of research to discover that the 100nF ones are used to filter high-frequency noise. If I have problems with that in the future I'll add them.

Monday, 11 May 2015


Interrupted is a new approach to developing low-power, low-latency Arduino sketches, which was motivated by two observations:
  • That the default mode of programming for Arduino is busy-waiting, and
  • The runtime support for power management provided by avr-libc is basic, being charitable to it.
Taken together, these two points mean that Arduino sketches aren't easily battery-powered, while the second results in a cargo-cult of code copying. Lastly, timed sleep in Arduino is provided by delay(), which itself is busy-waiting. This increases latency, the response time to external events such as button-presses.

And so to an example; the sketch below manages a very simple hardware configuration composed of an ATtiny85, an LED and a button: the button turns the LED on and a timer turns it off again. (You could imagine such a sketch driving a Useless Machine.)

In the main loop, the CPU sleeps in select(), waking up when an event occurs on one of the devices it manages. Three events are possible here:
  • An external interrupt which occurs when the button drives pin PB2 low,
  • A pin-change when the LED on PB0 is turned on or off,
  • When the watchdog timer fires, one second after the LED comes on.
The value returned from select() indicates which event has woken it up, allowing the concise construction of state-machines. If more than one event source is ready, the one added earlier in setup() is returned, implementing a crude form of priority.

The sleep-mode entered by select() is the maximum which makes sense for the devices currently active. In this little example, SLEEP_MODE_PWR_DOWN will be chosen because external and watchdog interrupts still work in this mode.

Much credit is due to Nick Gammon, for the great work contained in his Interrupts and Power Saving Techniques for Microprocessors forum-pages.

Very similar techniques were employed in the runtimes of the Conic and Regis environments developed at Imperial College (as well as the select system call, of course).

The library has been ported to ATtiny84, ATtiny85 and ATMega328 processors and is, as always, at GitHub. Its code footprint varies, depending on the functionality required by individual sketches, but the little example above comes in at 2.5kB.

Work remains to be done on this library including porting it to Energia (i.e., the msp430 family), porting existing sketches to use it and completing the repertoire of supported devices (e.g., SPI handling).

Wednesday, 11 March 2015

A tiny ATtiny

Very tiny!
 Recently I was surprised to discover that although I've made several projects using one, I'd never built a development board around the ATtiny85. With any project there are always design choices and a couple of important ones here were:
  • Is a breadboard optional or required? If optional, you get the familiar Arduino form-factor, which can either be connected to a breadboard using jumper cables or populated with pre-built shields.
  • Design a PCB or something more ad hoc? I wanted something quickly --- I wasn't going into competition with the Teensy or Trinket.
  • If not designing your own PCB, use stripboard or protoboard? While I've never been a huge fan of protoboard, because you're basically making your own strips by blobbing solder across pads, you can get quite a high component density with it.
So I opted for the path not taken, requiring a breadboard and using protoboard, and the result is pictured above. Notice that protoboard allows the FTDI connector to be attached at right-angles to the chip (try that with stripboard!) and also exposes more of the breadboard for jumper cables (the 10k resistor connecting reset and Vcc is underneath the chip).

With the ever-useful Tiny-Safe-Boot bootloader and a bespoke entry in my arduino-tiny's boards.txt file, it's as easy as regular Arduino, at a fraction the cost!

Thursday, 19 February 2015

TinySensor Server

Well after almost a year I decided it was time to update this project with improvements to both its hardware and software.

On the hardware side, the server is still the original Raspberry Pi model B, but I've mounted the nRF24L01+ radio on a neat little Slice of Pi board. (These boards also have traces for mounting XBees with their unusual pin spacing. And they're cheap!)

The Slice of Pi board (left) with radio and status LEDs
While I had the soldering iron out, I added a couple of status LEDs (see picture). The green one indicates recent activity on the radio, while the red one indicates a timeout but also flashes every time a packet is received. Both of them are driven by a single GPIO line on the Pi. (This was motivated by the radio cutting out on the Pi, probably due to the original Heath-Robinson jumper cable arrangement, and since its transplant it's behaved perfectly.)

On the software side, there are now quite a few little daemons, each doing one thing well:
  • A wireless hub, running RF24-rpi, sends packets received on a TCP socket,
  • A wired hub, running remotely, polls a legacy 1-wire sensor network and sends its readings on another TCP socket,
  • A multiplexor, connected to the two data sources above, sends their streams to its connected clients, described next,
  • A MySQL client writes readings to a database,
  • A status client manages the LEDs described above,
  • An RRD client maintains a round-robin database,
  • An lcdproc client updates a wireless LCD display.
All of these are, as usual, on GitHub.

Future software components planned for this system include a simple weather service to inject local weather data into the stream, and a client for one of the various IoT services now becoming widespread. (Here is a fairly recent overview of some of these.)

Wednesday, 3 December 2014

Space Invaders!

When I was a teenager Space Invaders arrived and changed the world. I must have put pounds into that machine in the local chipper, in the days when chips cost shillings. It never occurred to me that you could actually buy one of your own until one of my brother's friends' fathers did. (Come to think of it though, if I'd kept all of the cash I put into various machines, I probably could have had one of my own. I believe this is known as delayed gratification.)

Same Hardware, Different Processor!

Fast-forward 30-odd years and I'm sitting there blogging about an emulation of an 8-bit microcomputer of similar vintage. Hmm...

Space Invaders hardware comprises an Intel 8080 clocked at 2MHz, 8k ROM, 1k RAM and 7k memory-mapped video RAM. (An Emutalk thread has all the details and the ROMs.) The most difficult part is definitely the CPU but that only has to be done once to open up a whole world of crap games.

There are dozens of 8080 emulations out there, of varying quality. (Most of them written solely for this game!) I picked one and remixed it in the spirit of the 6502 emulator I made for the UK101; it plugs into that framework quite easily.

When a moderately-complex project like this links for the first time, the natural thing to do is upload it, more in hope than expectation, and when it fails, start debugging. The best way to do this is with a test-suite, preferably one which someone else has written in advance. (This worked out really well for my 6502 core thanks to Klaus Dormann's excellent 6502 tester.)

For the 8080, all searches led to a 'CPU exerciser'. Rather than testing a single instruction in isolation, this program runs a group of operations with different arguments and merging the resulting CPU state into a checksum as it goes. If the final checksum obtained by 'your' CPU is the same as one from a real chip, you know (to a high probability anyway) that your implementation is good for the instructions tested. (Luckily, that page documents results from real CPUs so there's no need to dig up an old 8080 board.)

If that sounds daunting, it is! Fortunately a preliminary test is also provided which tests the instructions required to run the exerciser itself. This helped fix a few bugs on its own and, since it used the CP/M "bdos conout" operation to report success, raised the interesting possibility of implementing a virtual 8080-based CP/M machine. I subsequently found a slightly more complex set of tests in another emulator's repository which helped find enough bugs to get all-but-two of the exerciser's tests passing.

(Timings of the exerciser indicate that my 8 year-old Lenovo laptop runs an emulated 8080 about 80x faster than the 2MHz 8080 for which it was intended. I haven't timed it on a Stellaris Launchpad however.)

Adventures in Dremel

So, despite the fact that some bugs remain in the emulation, enough of it is working to run Space Invaders properly. It runs at correct speed because it is driven by a timer interrupt (emulated with Energia's millisecond timer) and therefore we didn't need to bother keeping track of individual instruction timings, which all of the CPU emulators I looked at did. (Most of them were similar in other ways too, indicating a common influence, perhaps in this document.)

Just like the real thing, if you're 6" tall.

You can find the whole thing at Github, as usual.

Wednesday, 12 November 2014

In the Beginning... was the Command Line

Say what you like about the Arduino IDE, it's certainly lowered the barriers to entry for kids messing about with microcontrollers. 

Oh wait, that's not what I wanted to say at all...

Sometimes the Arduino IDE is simply not enough. It just gets in your way. Your project has outgrown it.

These were a few of my more printable thoughts as I tried to force a quart of UK101 emulation into the pint-pot of that tiny editor window. At times I'd no idea which file I was even editing! I longed to bust out emacs (or even vi).

What I really wanted was make. In particular GNU make. Simply the best build-tool ever, no question.

Many people have written Makefiles to build Arduino sketches but none of them felt right to me. They were all too complicated, too monolithic. Or maybe it was just a case of not-invented-here. In any case, I present uC-Makefile, which supports Arduino-1.0 and Energia.

The best thing about it so far has been control over the compiler's optimisation levels. Compiled with optimisation -O3, that old microcomputer's emulated Basic interpreter ran almost 5x faster!

Bye-bye, Arduino IDE!

Wednesday, 20 August 2014

Retrocomputer Resurrection

"What's a Computer?"
This chimera is a microcontroller emulation of a UK101, an 8-bit microcomputer from the early 1980s. Needless to say I had one; it is a shocking thirty-three years since, as a spotty teenager, I soldered one together over the course of a few days. I recall the kit cost the equally shocking sum of £99.95 (a lot of pocket-money), and had to be smuggled through Irish customs in my parents' car because the Single European Market was at that time merely a twinkle in M. Delors' eye. In its original configuration it had 1k of user RAM, 1k of display RAM, 8k ROM Basic (from Microsoft) and a 2k ROM monitor. When it went to its present resting place in the attic a couple of years later, it had 16k of user RAM, 2k of display RAM and an additional 6k of utility ROM, all piggy-backed on the original chips.

The last time I had patently too much time on my hands, in 1997 (seventeen years!), I hacked together a software simulation of it. I had intended this to be a general purpose simulator but it fell by the wayside after the UK101. In the intervening time I'd often regretted not writing the simulator in Java (indeed, a very good Java-based simulator now exists) but luckily where microcontrollers are concerned, C/C++ is still the only game in town, and most of the code compiled under the Energia IDE.

The hardware is almost insultingly simple, there are five components:
  • A Stellaris Launchpad from Texas Instruments,
  • An SD card drive,
  • A TFT LCD boosterpack,
  • A PS/2 socket,
  • A PS/2 keyboard.
While the Launchpad has only 16k of RAM, of which 12k is used for the main memory and 2k for the display, it does have 256k of flash, easily enough to contain all of the ROMs I've collected for it. (Another UK101 hardware instantiation can support the full 40k of user RAM.)

This build's main limitations are the display (only 30 lines of 40 characters vs the original's 32x48 ---  affordable TFT screens max-out at 240x320 pixels these days) and the keyboard (a lot of the games use the Control, Left-shift and Right-shift keys, which aren't available individually from the off-the-shelf PS/2 keyboard library).

However it has one modern advantages over the original which almost makes it enjoyable to use: checkpointing and restoring the machine state using the SD card. This allows creation of initial game images (which load instantly since they're only 14k) and progressive checkpoints, as adventure games are completed, for instance, or new programs developed.

Space Invaders: it doesn't get much better than this!
Code and programs are at GitHub, should you care.

The final thing is to find a suitably retrotacular housing for it. Something like this would be about right...

Kim Greist in "Brazil" by Terry Gilliam